Note 1 - Significant Accounting Policies and Concentrations of Risk - Fair Value of the Company's Investments by Type (Details) - USD ($) $ in Thousands	Dec. 31, 2020	Dec. 31, 2019
Amortized Cost	$ 28,211	$ 21,899
Accrued Interest	25	116
Estimated Fair Value	28,239	22,054
Gross Unrealized Gains	6	40
Gross Unrealized Losses		(1)
US Government Agencies Debt Securities [Member]
Amortized Cost	24,986	10,488
Accrued Interest	14	50
Estimated Fair Value	25,000	10,561
Gross Unrealized Gains		23
Certificates of Deposit [Member]
Amortized Cost	3,225	1,669
Accrued Interest	11	7
Estimated Fair Value	3,239	1,683
Gross Unrealized Gains	$ 3	7
Corporate Debt Securities [Member]
Amortized Cost		9,742
Accrued Interest		59
Estimated Fair Value		9,810
Gross Unrealized Gains		10
Gross Unrealized Losses		$ (1)